Most of spare fuel tanks disposed either as a separate type, outside the main fuel tank, or as a built-in type, inside the same or integrally constructed therewith. The separate type is defective in that it must be supplied fuel independently of the main tank and that it must be constantly checked to see if it is empty as it has no connection with the main tank. In some cases it communicates with the main tank, but it must then be more carefully checked for fear of being inadvertently emptied. The latter type appeared as an advanced type, and is widely used today. In one bottom corner of a main tank a certain space is separately occupied, in this type, for a spare tank by means of secluding or separating plates welded to the main tank. This advanced built-in type is apparently very convenient and practical in that it does not need individual or dual supplying of fuel because of automatic and simultaneous fuel supplying to both, and that it dispenses with the communication through piping between both tanks.